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NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 89
the other species. These characters alone separate diegensis from the
Modiolus group. A. diegensis is free-living, and therefore somewhat different from the other species, but the free-living habit alone is not sufficient to separate a species from its relatives in a genus.
Species of the genus Adula are distributed from Peru to British
Columbia, and Japan. The genus is related to Lithophaga Roding, Zeli-
thophaga Finlay, Terua Dall, Bartsch, and Rehder, and perhaps to
Botulopa Iredale.
Key to the west American species:
1. Shell with irregular vertical or wrinkled
sculpture, margins without crenulations falcata (Gould)
Shell smooth or with radiating sculpture 2
2. Radiating sculpture on the anterior part
and before the keel, margins crenulated
anteriorly and strongly so behind the
ligament soleniformis (Orbigny)
Shell smooth or with few radiating
striae anteriorly, posteriorly curved,
with a filthy incrustation 3
3. Shell elongate, not higher posteriorly
than anteriorly, lunule generally without radiating lines californianus (Philippi)
Shell shorter, distinctly highest posteriorly, lunule generally with distinct
radiating lines, which form internal
ribs diegensis (Dall)
Adula falcata (Gould) 1851
Plate 9, fig. 49
Lithodomus falcatus Gould, Proc. Boston Soc. Nat. Hist., vol. 4, 1851,
p. 92.
Holotype: ?
Type loc: Monterey, California.
Remarks: This species has the umbones near the anterior end. The
peculiar sculpture of vertical or oblique or partly irregular wrinkles is
typical for the species. The posterodorsal triangle has a filthy incrustation. The scars of the siphonal retractors consist of many small impressions just below the posterior adductor. The ventral siphon has irregular

NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 89
the other species. These characters alone separate diegensis from the
Modiolus group. A. diegensis is free-living, and therefore somewhat different from the other species, but the free-living habit alone is not sufficient to separate a species from its relatives in a genus.
Species of the genus Adula are distributed from Peru to British
Columbia, and Japan. The genus is related to Lithophaga Roding, Zeli-
thophaga Finlay, Terua Dall, Bartsch, and Rehder, and perhaps to
Botulopa Iredale.
Key to the west American species:
1. Shell with irregular vertical or wrinkled
sculpture, margins without crenulations falcata (Gould)
Shell smooth or with radiating sculpture 2
2. Radiating sculpture on the anterior part
and before the keel, margins crenulated
anteriorly and strongly so behind the
ligament soleniformis (Orbigny)
Shell smooth or with few radiating
striae anteriorly, posteriorly curved,
with a filthy incrustation 3
3. Shell elongate, not higher posteriorly
than anteriorly, lunule generally without radiating lines californianus (Philippi)
Shell shorter, distinctly highest posteriorly, lunule generally with distinct
radiating lines, which form internal
ribs diegensis (Dall)
Adula falcata (Gould) 1851
Plate 9, fig. 49
Lithodomus falcatus Gould, Proc. Boston Soc. Nat. Hist., vol. 4, 1851,
p. 92.
Holotype: ?
Type loc: Monterey, California.
Remarks: This species has the umbones near the anterior end. The
peculiar sculpture of vertical or oblique or partly irregular wrinkles is
typical for the species. The posterodorsal triangle has a filthy incrustation. The scars of the siphonal retractors consist of many small impressions just below the posterior adductor. The ventral siphon has irregular